The binary icosahedral group is most easily described concretely as a discrete subgroup of the unit quaternions, under the isomorphism where Sp(1) is the multiplicative group of unit quaternions. (For a description of this homomorphism see the article on quaternions and spatial rotations.)

In total there are 120 elements, namely the unit icosians. They all have unit magnitude and therefore lie in the unit quaternion group Sp(1). The convex hull of these 120 elements in 4-dimensional space form a regular 4-polytope, known as the 600-cell.

This sequence does not split, meaning that 2I is not a semidirect product of { ±1 } by I. In fact, there is no subgroup of 2I isomorphic to I.

The center of 2I is the subgroup { ±1 }, so that the inner automorphism group is isomorphic to I. The full automorphism group is isomorphic to S5 (the symmetric group on 5 letters), just as for - any automorphism of 2I fixes the non-trivial element of the center (), hence descends to an automorphism of I, and conversely, any automorphism of I lifts to an automorphism of 2I, since the lift of generators of I are generators of 2I (different lifts give the same automorphism).

The binary icosahedral group is perfect, meaning that it is equal to its commutator subgroup. In fact, 2I is the unique perfect group of order 120. It follows that 2I is not solvable.

Further, the binary icosahedral group is superperfect, meaning abstractly that its first two group homology groups vanish: Concretely, this means that its abelianization is trivial (it has no non-trivial abelian quotients) and that its Schur multiplier is trivial (it has no non-trivial perfect central extensions). In fact, the binary icosahedral group is the smallest (non-trivial) superperfect group.

The binary icosahedral group is not acyclic, however, as Hn(2I,Z) is cyclic of order 120 for n = 4k+3, and trivial for n > 0 otherwise, (Adem & Milgram 1994, p. 279).

Concretely, the binary icosahedral group is a subgroup of Spin(3), and covers the icosahedral group, which is a subgroup of SO(3). Abstractly, the icosahedral group is isomorphic to the symmetries of the 4-simplex, which is a subgroup of SO(4), and the binary icosahedral group is isomorphic to the double cover of this in Spin(4). Note that the symmetric group does have a 4-dimensional representation (its usual lowest-dimensional irreducible representation as the full symmetries of the -simplex), and that the full symmetries of the 4-simplex are thus not the full icosahedral group (these are two different groups of order 120).

The binary icosahedral group can be considered as the double cover of the alternating group denoted this isomorphism covers the isomorphism of the icosahedral group with the alternating group and can be thought of as sitting as subgroups of Spin(4) and SO(4) (and inside the symmetric group and either of its double covers in turn sitting inside either pin group and the orthogonal group ).

Unlike the icosahedral group, which is exceptional to 3 dimensions, these tetrahedral groups and alternating groups (and their double covers) exist in all higher dimensions.

Note also the exceptional isomorphism which is a different group of order 120, with the commutative square of SL, GL, PSL, PGL being isomorphic to a commutative square of which are isomorphic to subgroups of the commutative square of Spin(4), Pin(4), SO(4), O(4).

is the only element of order 2, hence it is contained in all subgroups of even order: thus every subgroup of 2I is either of odd order or is the preimage of a subgroup of I. Besides the cyclic groups generated by the various elements (which can have odd order), the only other subgroups of 2I (up to conjugation) are:

The 4-dimensional analog of the icosahedral symmetry groupIh is the symmetry group of the 600-cell (also that of its dual, the 120-cell). Just as the former is the Coxeter group of type H3, the latter is the Coxeter group of type H4, also denoted [3,3,5]. Its rotational subgroup, denoted [3,3,5]+ is a group of order 7200 living in SO(4). SO(4) has a double cover called Spin(4) in much the same way that Spin(3) is the double cover of SO(3). Similar to the isomorphism Spin(3) = Sp(1), the group Spin(4) is isomorphic to Sp(1) × Sp(1).

The preimage of [3,3,5]+ in Spin(4) (a four-dimensional analogue of 2I) is precisely the product group 2I × 2I of order 14400. The rotational symmetry group of the 600-cell is then

[3,3,5]+ = ( 2I × 2I ) / { ±1 }.

Various other 4-dimensional symmetry groups can be constructed from 2I. For details, see (Conway and Smith, 2003).